Strain-induced transformation to ultrafine microstructure in steel

ABSTRACT

Steel with ultrafine grains is produced by altering the transformation from one which normally proceeds with grain boundary nucleation followed by intragranular nucleation at deformation bands and other defects, to one which induces a substantially instantaneous transformation homogeneously over the austenite grain. This is favoured by a reduction or minimisation of grain boundary nucleation, (for example by enlargement of the austenite grain size), prior to or during the transformation. In an embodiment, a partially cooled austenite phase steel is deformed in a single pass at a temperature in the range of 700-950° C. to obtain ferrite grain size of 5 μm or less.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to the production and processing of steels toachieve ultrafine microstructures. For example, in a ferrite containingsteel, ultrafine microstructures are considered to be those having asignificant proportion of grains of a size less than 5 microns in aplain carbon steel, or less than 3 microns in a microalloyed steel.

2. Related Art

One of the principal aims of modem steel processing methods is to refineferrite grain size. A small ferrite grain size is desirable as thisresults in a steel with improved strength and toughness.

In recent years, there have been several reports in the scientificliterature of different techniques for producing low carbon microalloyedsteels with ultra fine ferrite grains. One type of approach has reliedupon the expectation that dynamic recrystallisation at temperatures onlya little above the austenite to ferrite transformation temperature (Ar₃)will produce a small grain size. Controlled rolling schedules have thusbeen devised, using laboratory simulation by torsion or compressiontesting, which exploit dynamic recrystallisation after strainaccumulation.

In one case, Kaspar et al reported production of austenite grains downto 1 to 4 micron in a compression tested Nb-V microalloyed steel whichtransformed on cooling to ferrite with a mean grain size less than 5micron ["Thermec 88" Proc.Int.Conf. on Physical Metallurgy ofThermomechanical Processing of Steels and Other Metals, I.S.I.J. 1988,2, 713]. Samuel et al reported that torsion testing of niobiummicroalloyed steels produced austenite and ferrite grain sizes of 5 and3.7 micron, respectively, in deformation schedules where strainaccumulation from successive passes led to dynamic recrystallisation[I.S.I.J. Int., 1990, 30, 216].

U.S. Pat. No. 4,466,842 to Yada et al describes a hot-rolled ferriticsteel composed of 70% or more of equiaxed ferrite grains having anultra-fine grain size of 4 μm or less. This steel is produced by hotworking at approximately the Ar₃ point and by one or more passes of hotworking having a minimum required total reduction ratio of at least 75%.Due to hot working, dynamic transformation of austenite and/or dynamicrecrystallisation of ferrite takes place.

For plain carbon steels, Matsumura and Yada [I.S.I.J. 1987, 27, 492 and"Thermec 88" I.S.I.J. 1988, 1, 200] disclosed hot working schedulesusing laboratory compression and rolling tests to produce ferrite grainsizes below 3 micron. By imposing large strains just above the Ar₃, theyinduced transformation during the deformation (despite the increase intemperature from the heat of deformation) and then continued to work theferrite sufficiently for it to recrystallise dynamically. Rapidquenching after the deformation, while preventing coarsening of theferrite grains, led to some martensite formation. By imposing strains upto 4, microstructures with 70-80% ferrite as fine as 1 to 2 micron wereproduced. Reducing the amount of intercritical deformation tended toreduce the volume fraction of ferrite and to increase the mean grainsize.

Other techniques to produce ultrafine grains have been more involved.Ameyama et al. ["Thermec 88", I.S.I.J. 1988, 2, 848] disclosed lowtemperature deformation and brief austenitising cycles, combined withthe addition of 3% Mn and 1% Mo to enhance austenite nucleation onreheating, to produce austenite grain sizes down to 1 micron indiameter. Kurzydlowski et al. [Z. Metallkunde, 1989, 80, 469] alsodisclosed repeated cold deformation and anneal cycles, together withboron additions, to produce austenitic stainless steels with grain sizesdown to 1 micron diameter. Although these methods are of considerablescientific interest, they are a relatively expensive means of producingultrafine grains.

More recently, Beynon et al have reported [Materials Forum 1992, 16, 37]the production of ultrafine Nb microalloyed ferrite, with an averagegrain size of approximately 1 micron, using laboratory hot torsiontests. The tests utilised controlled hot deformation at a temperature ofabout 1050° C., followed by rapid cooling through a sequence of six toeight finishing deformations, starting at 900° C. Each deformation wasto a strain of 0.3 at an equivalent uniaxial strain rate of 2.3/s, andthe final deformation was close to Ar₃, when maximum refinement wasobserved. The finest structure produced was a uniformly fine equiaxedferritic microstructure with approximately 5% pearlite and a mean grainsize for the ferrite of 1.3 micron. It was proposed that the refinementwas due to strain induced transformation of a heavily controlled rolledinitial austenite microstructure, in which the deformation increases thedensity of the nucleation sites for transformation to ferrite. Such amechanism of ferrite refinement had been reported in the Matsumura andYada paper first listed above. Priestner ["Thermomechanical Processingof Microalloyed Austenite", Met.Soc.A.I.M.E., 1981, 455] also obtainedfine grains in regions of laboratory rolled samples which transformed inthe roll gap during rolling. Again a large strain was necessary and thetransformed product was mixed and quite "patchy", with some very largegrains present. The processes reported by Beynon et al and by Priestnerare again of scientific rather than practical interest.

It is a first preferred object of the invention to provide a practicalprocess for the production of steels with ultrafine microstructures inany of a variety of phases or mixtures of phases, including eg bainite.

It is a second preferred object of the invention to provide a practicalprocess for the production of steels with ultrafine ferritemicrostructures.

It is a third preferred object of the present invention to provide asteel with an ultrafine microstructure, particularly an ultrafineferrite microstructure.

It is a fourth preferred object of the present invention to provideapparatus for use in the production of steels with ultrafine ferritemicrostructure.

SUMMARY OF THE INVENTION

The present invention stems from an initial surprising discovery that anaustenite to ferrite transformation which achieves ultrafine ferritegrains can be achieved by the single deformation of a steel having largeaustenite grains, e.g. greater than 80 micron. This is quite contrary tothe normal expectation that the smaller the size of the ferrite grainssought in the end product, the smaller the size of the austenite grainsrequired prior to the transformation. The invention is not justperceived in terms of the specific context of this discovery. Rather, itis more broadly appreciated that steels with ultrafine ferrite grainsmay be produced by altering the transformation from one which normallyproceeds with grain boundary nucleation followed by intragranularnucleation at deformation bands and other defects, to one which inducesa substantially instantaneous transformation to ferrite homogeneouslyover the austenite grain. This is favoured, for example, by a reductionor minimisation of grain boundary nucleation of the ferrite grains priorto or during the transformation. Enlargement of the austenite grain sizeis of course one means of reducing grain boundary nucleation since itentails reduction of grain boundaries, but other methods may beemployed.

It has also been appreciated that when a partially cooled austenitephase steel is deformed in a single pass in a temperature in the rangefor example of 700 to 950° C., the transformation to ferrite does notoccur prior to deformation, as would conventionally be expected, butinstead takes place rapidly during or immediately following thedeformation.

It has also been appreciated that an austenite to ferrite transformationwhich achieves ultrafine ferrite grains can be achieved by austenitisinga steel to a large grain size and then partially cooling and deformationtreating the steel in the austenite phase. This is quite unexpectedgiven the conventional wisdom that the reheating of a steel to give acoarse austenite grain size phase will then result in a coarse ferritegrain size after transformation on cooling.

It has further been found that the invention is not confined to theproduction of an ultrafine ferrite microstructure but is able to produceultrafine microstructures in any of a variety of phases or mixtures ofphases, including e.g. bainite.

The invention accordingly provides, in a first principal aspect, amethod of producing a steel having one or more zones of ultrafinemicrostructure comprising treating an austenite phase steel before anysubstantial transformation has commenced so as to induce a rapidsubstantially complete transformation to an ultrafine microstructure inone or more zones of the microstructure.

In a second principal aspect the invention comprises a method ofproducing a steel having one or more zones of ultrafine microstructurecomprising heating a steel to austenitise the steel, pre-cooling theaustenite phase steel, treating the austenite phase steel before anysubstantial transformation has commenced so as to induce a rapidsubstantially complete transformation to an ultrafine microstructure inone or more zones of the microstructure.

The pre-cooling of the austenite phase steel is preferably by naturalair, forced air or water cooling at a rate in the range 50 to 2000K/min.

In a third principal aspect the invention comprises a method ofproducing a steel having one or more zones of ultrafine microstructurecomprising partially pre-cooling freshly cast austenite phase steel,treating the austenite phase steel before any substantial transformationhas commenced so as to induce a rapid substantially completetransformation to an ultrafine microstructure in one or more zones ofthe microstructure.

As employed herein, the term "austenite phase steel" refers to a steelwhich is in the austenite phase. It is appreciated that some steels,such as freshly cast steel, may have a number of other phases formedtherein prior to reaching the austenite phase.

Preferably, the treatment applied to the austenite phase steel is adeformation performed at a temperature in the range of 600° C. to 950°C., more preferably 700° C. to 950° C. for a low carbon steel.

In a fourth principal aspect the invention comprises a method ofproducing a steel having one or more zones of ultrafine microstructurecomprising deforming an austenite phase steel before any substantialtransformation has commenced to so develop a predetermined strainprofile or strain gradient across the structure of the steel so as toinduce a rapid substantially complete transformation to an ultrafinemicrostructure in one or more zones of the microstructure.

Preferably, the zone of the ultrafine microstructure comprises a wholecross-section of the structure, most preferably a uniform ultrafinemicrostructure. In an alternative embodiment, the zones of the ultrafinemicrostructure may comprise a surface layer or layers of the steel. Forthe latter purpose, in the fourth aspect of the invention, thepredetermined strain profile may comprise a relatively higher strain ina surface layer or layers of the steel and a relatively lower strain inthe core. The transformation to the ultrafine microstructure then tendsto occur in the surface layer or layers. This strain inhomogeneity canbe enhanced by having friction conditions existing between the surfaceof the steel being rolled (ie the strip surface) and the roll.Alternatively, in the fourth aspect, by manipulating the coefficient offriction between the surface of the steel being rolled and the roll, asteel may be achieved in which a whole cross-section of the structure istransformed to an ultrafine microstructure, preferably a substantiallyuniform ultrafine microstructure.

In this context, the term "strain profile" preferably refers to aneffective strain profile, where the effective strain encompasses thecombined effect of shear strain due to the contact between the strip andthe roll, and the compressive strain which relates to the simplereduction in thickness.

The deformation applied to the austenite phase steel, as with otheraspects of the invention advantageously comprises deformation rolling.The rolling speed is preferably in the range 0.1 to 5.0 m/s. To developthe preferred strain profile the ratio of rolling arc (L_(d)) to nip gapor rolling thickness (H_(m)) is preferably greater than 10.

As employed herein, the term "rapid substantially completetransformation" indicates 90% transformation to the final ultrafinemicrostructure within the deformation zone or within one second ofdeparture therefrom. In the case of a ferrite product, it will beunderstood that the transformation to ferrite is a rapid substantiallycomplete transformation, whereas the carbide (cementite) formation mayoccur over a longer time frame. In the case of a bainite product, theentire transformation may occur in the deformation zone or within onesecond of departure therefrom.

The deformation in any of the first, second, third or fourth aspects ofthe invention preferably includes, and most preferably solely comprises,passing the steel between a pair of contra-rotating rolls effective toreduce a thickness dimension of the steel by a proportion in the range20 to 70%, most preferably 30 to 60%, to a value defined by the loadednip between the rolls. Preferably, only a single deformation of thesteel is performed, eg a single pass of the steel between a pair ofcontra-rotating rolls. With rolls, the aforementioned deformation zonecomprises the arc of contact between the steel and the rolls,terminating at the nip. The roll geometry, e.g. the rate of rolling, orroll diameter relative to steel thickness, may be selected to optimisesaid rapid substantially complete transformation. There may of course befurther roll treatments prior to or subsequent to the transformation,but it is preferred that, prior to the deformation, the austenite phasesteel has not been worked or has been worked only lightly.

Preferably, the deformation induces a largely homogenous transformationto an ultrafine microstructure. The transformation preferably occursmostly during the deformation process, although some transformation maytake place soon after the deformation. The transformation to theultrafine microstructure is preferably complete within one second afterthe deformation. This transformation process is being called a "straininduced transformation".

In accordance with the second aspect of the invention, the steel ispreferably heated to a temperature between 1000° C. to 1400° C., mostpreferably in the range 1100° C. to 1300° C.

Preferably, in each of the first, second, third and fourth aspects thesteel is cooled after the transformation.

The ultrafine microstructure may comprise, for example, ultrafinepredominantly ferrite grains, or, by way of further example, it may be abainite microstructure.

Preferably, the austenite phase steel has a mean austenite grain sizegreater than 50 micron, more preferably greater than 80 micron. Theaustenite grain size in traditional hot rolled steel prior totransformation is around 40 micron. The austenite phase steel may beequiaxed.

In addition to, or instead of the austenite phase steel having anaustenite grain size in the aforementioned preferred range, the steelmay be pretreated in a manner effective to reduce or substantiallyeliminate grain boundary nucleation of ferrite grains, whereby tofacilitate said rapid transformation. Such pretreatment may comprise apretreatment to enlarge the mean austenite grain size of a selectedsteel or may alternatively or additionally comprise a chemicaltreatment, for example the addition of a component (e.g. boron) selectedto reduce grain boundary reactivity. The pretreatment may advantageouslyentail a pre-cooling of the steel from a higher temperature, for examplein the range 1000 to 1400° C., to the aforesaid temperature range, 600°C. to 950° C.

It is thought that the cooling of the transformed steel need not beparticularly rapid and thus may be effected by forced air cooling, forexample to produce a cooling rate up to 500° K/min, preferably between50 and 2000° K/min. Of course, the invention does not preclude a sloweror more rapid cooling if this proves to be beneficial. A particularembodiment of the invention may involve back spraying of cooling fluidinto the roller nip to modify the grain size, e.g. the ferrite grainsize, at the surface of the transformed steel.

The steel subjected to the deformation is preferably steel strip, plate,sheet, rod or bar, although the invention is also applicable to othersteel forms, e.g. billet or slab. The strip, plate, sheet, rod or bar ispreferably of a thickness less than 20 mm, most preferably less than 10mm. It is thought that the invention is primarily applicable to producewhat is conventionally regarded as thin strip (<5 mm) because it is insuch strip that the distribution of the ultrafine microstructure can beoptimised.

In a fifth principal aspect, the invention provides a steel with anultrafine microstructure, for example having ultrafine ferrite grains,which is uniform and at least partially ultrafine in one or more zonesand has a mean grain size no greater than 3 micron in these zone(s).Preferably, the steel has a mean grain size at the centre≦10 micron andin the surface layer(s)≦2 micron. Most preferably, a substantialproportion of the volume of a ferrite grain microstructure, for exampleat least 30%, may contain ferrite grains primarily of a size less than 3micron. The microstructure of the steel may be layered, for example asurface layer or layers having zones of ultrafine microstructure, and acore layer of relatively coarse microstructure. Preferably, in thislayered microstructure, at least 80% by volume of the fine grained layercontains grains primarily of a size less than 3 micron.

The deformation temperature may be selected in accordance with thedesired end product steel specification, e.g. a higher deformationtemperature for a softer steel.

Typically, said deformation will be accompanied by some cooling of thesteel, for example by providing a conduction path for heat. This mightbe enhanced in the known manner by the use of lubricant and/orpositively cooled rolls.

Preferably, where the product steel is a ferrite phase, thetransformation to ferrite is such as to produce a microstructure inwhich the mean ferrite grain size at the centre of the steel is no morethan 10 times the mean grain size in the surface layer.

The ultrafine microstructure is typically equiaxed, but this is not ofcourse essential.

The steel may be pretreated eg by preheating and partial cooling toincrease the proportion of grains which transform to said ultrafinemicrostructure.

The austenite phase steel is preferably a low carbon (C<0.3%) steel, andmay be a low carbon microalloyed steel. However, higher carbon steelshave also been shown to behave in the same manner, and can formultrafine structures when processed according to this invention.

In a sixth principal aspect the invention comprises a combinationcasting and deformation apparatus for producing a steel having one ormore zones of ultrafine microstructure comprising means to cast anaustenite phase steel, means disposed to receive and partially pre-coolthe freshly cast austenite phase steel, and means to treat the partiallycooled steel before any substantial transformation has commenced so asto induce a rapid substantially complete transformation to an ultrafinemicrostructure in one or more zones of the microstructure.

The casting means may be a thin slab or strip caster and the treatmentmeans preferably includes rolling means, eg a single pair ofcontra-rotating rolls.

In a seventh principal aspect the invention comprises a deformationapparatus for producing a steel having one or more zones of ultrafinemicrostructure comprising means to heat the steel to the austenitephase, means to partially pre-cool the austenite phase steel, means totreat the partially cooled austenite phase steel before any substantialtransformation has commenced so as to induce a rapid substantiallycomplete transformation to an ultrafine microstructure in one or morezones of the microstructure.

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying drawings and examples in which:

FIG. 1 is a simple diagram of a compact rolling line in accordance withan embodiment of the sixth aspect of the invention;

FIG. 2 is a simple diagram of a combination strip reheating and rollingline in accordance with an embodiment of the seventh aspect of theinvention;

FIG. 3 is a simple diagram of a single pass rolling deformation used inan embodiment of the method of the fourth aspect of the invention;

FIG. 4 is a diagram of an exemplary cross-sectional strain profilethrough the strip of FIG. 3;

FIG. 5 illustrates the displacement of successive transverse segments ofthe microstructure of the strip shown in FIG. 3;

FIG. 6 is an optical micrograph showing the surface regions of ultrafinegrains of a steel in accordance with an embodiment of the invention;

FIG. 7A is a scanning electron micrograph of ultrafine ferrite grains insurface regions of M06 steel strip;

FIG. 7B is an optical micrograph of coarse ferrite grains in centreregions of M06 strip;

FIG. 7C is an optical micrograph of M06 sample rolled at low entrytemperature;

FIG. 8A is a scanning electron micrograph of ultrafine microstructure insurface regions of M06 strip rolled at low speed;

FIG. 8B is an optical micrograph of ferrite grains in surface regions ofM06 strip rolled at high speed;

FIG. 9A is an optical micrograph of surface region of M06 rolled withlubrication;

FIG. 9B is an optical micrograph of surface region of M06 rolled withoutlubrication;

FIG. 10A is a scanning electron micrograph showing carbide distributionin surface regions of M06 after air cooling;

FIG. 10B is a scanning electron micrograph showing carbide distributionin surface regions of M06 after coiling at 650° C.;

FIG. 11A is an optical micrograph showing ultrafine ferrite and carbidedistribution in surface regions of 0.065 C-0.99 Mn steel (3373);

FIG. 11B is an optical micrograph showing acicular ferrite in centreregions of 0.065 C-0.99 Mn steel (3373);

FIG. 12A is an optical micrograph showing ultrafine ferrite in surfaceregions of high SI steel (3398) after 1250° C. reheat;

FIG. 12B is an optical micrograph showing worked ferrite in surfaceregions of high SI steel (3398) after 950° C. reheat;

FIG. 13A is an optical micrograph showing ultrafine ferrite in surfaceregions of Ti microalloyed steel (3403);

FIG. 13B is an optical micrograph showing coarse ferrite and martensiteislands in centre regions of Ti microalloyed steel (3403);

FIG. 14A is an optical micrograph showing ultrafine ferrite in surfaceregions of Ti-Mo microalloyed steel (3403);

FIG. 14B is an optical micrograph showing acicular ferrite andmartensite islands in centre regions of Ti-Mo microalloyed steel (3404);

FIG. 15A is a scanning electron micrograph showing ultrafine ferrite insurface regions of high Ti steel (3394);

FIG. 15B is a scanning electron micrograph showing acicular ferrite andmartensite islands in centre regions of high Ti steel (3394);

FIG. 16A is an optical micrograph showing ultrafine ferrite and carbidesegregation in surface regions of 0.21 C-0.99 Mn steel (3374);

FIG. 16B is an optical micrograph showing necklacing and acicularferrite in centre regions of 0.21 C-0.99 Mn steel (3374);

FIG. 17A is an optical micrograph showing ultrafine ferrite and carbidesin surface regions of 1040 steel;

FIG. 17B is an optical micrograph showing pearlite and proeutectoidferrite in centre regions of 1040 steel;

FIG. 17C is a scanning electron micrograph showing carbide distributionin surface regions of 1040 steel after air cooling;

FIG. 17D is a scanning electron micrograph showing carbide distributionin surface regions of 1040 steel after coiling at 600° C.;

FIG. 18A is an optical micrograph showing carbide distribution insurface regions of 0.27 C-0.12 V steel (3524) after air cooling;

FIG. 18B is an optical micrograph showing carbide distribution insurface regions of 0.27 C-0.12 V steel (3524) after coiling at 600° C.;

FIG. 19A is an optical micrograph showing ultrafine ferrite in surfaceregions of Ti-B medium C steel (3605) after 1250° C. reheat;

FIG. 19B is an optical micrograph showing coarse, worked ferrite insurface regions of Ti-B medium C steel (3605) after 950° C. reheat;

FIG. 20A is a scanning electron micrograph showing ultrafine ferrite insurface regions of 1077 eutectoid steel;

FIG. 20B is a scanning electron micrograph showing pearlite in centreregions of 1077 eutectoid steel;

FIG. 21 is a stress-strain curve of low C steel (A06) displaying no workhardening; and

FIG. 22 is a stress-strain curve of high C steel (1062) displaying arelatively high level of work hardening.

FIG. 1 is a simple diagram of a combination strip casting and rollingline 10 comprising one embodiment of the sixth aspect of the invention.Austenite phase hot steel strip 11 of gauge preferably less than 10 mm,emerges vertically downwardly from a strip caster 12, and is feddirectly to a pre-cooler 16. Here, the steel is pre-cooled, by naturalair, forced air or water cooling, to a temperature in the range 700 to950° C. Still austenite phase, the strip is now presented for a singlepass 50% reduction at a roll stand 18 to so strain the steel as toinduce rapid substantially complete transformation. The transformedrolled strip 19, half its former thickness, is now passed through anatural air, forced air or water cooler 20 to cool it to ambienttemperature, or to a selected intermediate temperature. The ultrafinegrain steel strip is then gathered onto a coiler 22. Surface temperatureof the steel before and after the deformation zone, defined by the arcof contact at the rollers, is monitored by respective pyrometers 24,25.

FIG. 2 is a simple diagram of a compact rolling line 50 comprising oneembodiment of the seventh aspect of the invention. Steel strip 51 ofgauge preferably less than 10 mm is withdrawn from a coiler 52 andpassed through a furnace e.g. a transverse flux induction furnace 54 inwhich the strip is heated past the austenite phase equilibriumtemperature (Ae₃) to transform it to austenite. This austenite phasesteel 55 is pre-cooled to a temperature in the range 700° C. to 950° C.in a natural air, forced air or water pre-cooler 56. Still austenitephase, the strip 55 is now presented for a single pass 50% reduction ata roll stand 58 to so strain the steel as to induce rapid substantiallycomplete transformation. The transformed rolled strip 59, half itsformer thickness is now passed through a natural air, forced air orwater cooler 70 to cool it to ambient temperature, or to a selectedintermediate temperature. The ultrafine grain steel strip is thengathered onto a coiler 72. The surface temperature of the steel beforeand after the deformation zone, defined by the arc of contact at therollers, is monitored by respective pyrometers 74,75.

FIG. 3 diagrammatically depicts a cross-section of a single pass rollingdeformation suitable for practising the fourth aspect of the presentinvention with a strip 100. The rolls are at 112 and FIG. 3 alsoindicates the aforementioned parameters L_(d) and H_(m). FIG. 4 is adiagram depicting an exemplary cross-sectional strain profile throughthe strip thickness of the general form preferred in accordance withthis aspect of the present invention. The effective strain refers to thecombined effects of the reduction strain, given by 1 n H/h where H isthe strip thickness at the entry to the roll and h is the stripthickness at the roll exit, and the shear strain due to the frictionconditions. FIG. 5 illustrates the displacement of successive transversesegments 105 of the metal in a longitudinal cross-section of the stripthrough the deformation zone at a given time point. FIG. 6 depicts atypical resultant layered microstructure (ie surface layers ofpredominantly ultrafine microstructure and a core of relatively coarsermicrostructure). It will be seen that the width of the layerscorresponds to the high strain surface zones indicated in FIGS. 4 and 5.

EXAMPLE 1

Low carbon steel strip (C 0.09%, Mn 1.47% Si 0.08% Nb 0.027% Ti 0.025%,the balance Fe plus typical levels of residue elements) at a surfacetemperature of 1250° C. and having observed austenite grain sizesprimarily in the range 100 to 200 micron, was pre-cooled to a surfacetemperature of 800° C. by being left to naturally cool in air. Thecooled strip, of 2.25 mm thickness, was deformation rolled, in a singlepass through the nip of a pair of contra-rotating rolls, to cause a 38%reduction in thickness to 1.38 mm. The exit surface temperature of thesteel strip from the rolls was 700° C. The strip was then left to coolin air to ambient temperature.

The ferrite grain size varied between <1 and 12 micron, and asubstantial proportion of the total volume, about 60%, had grain sizesprimarily in the range <1 to 3 micron. These ultrafine zones wereconcentrated at or close to the surface. From observation, it was foundthat the partially cooled steel presented to the nip was not alreadypartially or wholly transformed, but was still substantially whollyaustenite phase steel. Moreover, it was thought that the austenite toferrite transformation occurred at or very close after the roller nip,suggesting that the mechanism was strain induced transformation. It wasalso observed that there was little or no tendency for the ferritegrains to thereafter coarsen despite the relatively slow rate of coolinginherent in natural air cooling, suggesting that the transformation wassubstantially instantaneous, whereby the grains were locked in positionagainst expansion in size.

EXAMPLE 2

Low carbon steel strip (C 0.1%, Mn 1.38%, Si 1.4%, the balance Fe plustypical levels of residue elements) at a surface temperature of 1250° C.and having observed austenite grain sizes primarily in the range 100 to200 micron, was pre-cooled to a surface temperature of 775° C. by beingleft to naturally cool in air. The cooled strip, of 2.13 mm thickness,was deformation rolled, in a single pass through the nip of a pair ofcontra-rotating rolls, to cause a 39% reduction in thickness to 1.3 mm.The exit surface temperature of the steel strip from the rolls was 688°C. The strip was then left to cool in air to ambient temperature.

The ferrite grain sizes varied between <1 and 20 micron, and asubstantial proportion of the total volume, about 60%, had grain sizesprimarily in the range <1 to 3 micron. These ultrafine zones wereconcentrated at or close to the surface. From observation, it was foundthat the partially cooled steel presented to the nip was not alreadypartially or wholly transformed, but was still substantially whollyaustenite phase steel. Moreover, it was thought that the austenite toferrite transformation occurred at or very close after the roller nip,suggesting that the mechanism was strain induced transformation. It wasalso observed that there was little or no tendency for the ferritegrains to thereafter coarsen despite the relatively slow rate of coolinginherent in natural air cooling, suggesting that the transformation wassubstantially instantaneous, whereby the grains were locked in positionagainst expansion in size.

EXAMPLE 3

Freshly cast steel as such was not readily available for the purposes ofexperimentation. However, a low carbon steel strip (C 0.07%, Mn 0.4%,the balance Fe plus typical levels of residue elements) at a surfacetemperature of 1250° C. was employed to simulate a freshly cast steel.The steel had austenite grain sizes primarily in the range 100 to 200micron. The steel used differed from fresh cast steel in that the grainstructure was equiaxed. The steel was pre-cooled to a surfacetemperature of 800° C. by being left to naturally cool in air. Thecooled strip, of 1.8 mm thickness, was deformation rolled, in a singlepass through the nip of a pair of contra-rotating rolls, to cause a 45%reduction in thickness to 1.0 mm. The exit surface temperature of thesteel strip from the rolls was 680° C. The strip was then left to coolin air to 600° C., at which temperature it was held for an hour tosimulate coiling, then left to cool in air to ambient temperature.

The product was found to be 95% ferrite, distributed uniformly in thestrip. The ferrite grain sizes varied between 1 and 10 micron, and asubstantial proportion of the total volume, about 60%, had grain sizesprimarily in the range 1 to 2 micron. These ultrafine zones wereconcentrated at or close to the surface. From observation, it was foundthat the partially cooled steel presented to the nip was not alreadypartially or wholly transformed, but was still substantially whollyaustenite phase steel. Moreover, it was thought that the austenite toferrite transformation occurred at or very close after the roller nip,suggesting that the mechanism was strain induced transformation. It wasalso observed that there was little or no tendency for the ferritegrains to thereafter coarsen despite the relatively slow rate of coolinginherent in coiling, suggesting that the transformation wassubstantially instantaneous, whereby the grains were locked in positionagainst expansion in size.

The strip was tested in tension and found to have a yield strength of460 MPa and an ultimate tensile strength of 480 MPa. The totalelongation was 28% and the uniform elongation was 20%.

EXAMPLE 4

Low carbon steel strip (C 0.1%, Mn 0.86%, Si 0.29%, Nb 0.037%, thebalance Fe plus typical levels of residue elements) at a surfacetemperature of 1250° C. and having observed austenite grain sizesprimarily in the range 100 to 200 micron, was pre-cooled to a surfacetemperature of 800° C. by being left to naturally cool in air. Thecooled strip, of 2.4 mm thickness, was deformation rolled, in a singlepass through the nip of a pair of contra-rotating rolls, to cause a 40%reduction in thickness to 1.43 mm. The exit surface temperature of thesteel strip from the rolls was 696° C. The strip was then left to coolin air to ambient temperature.

The ferrite grain sizes varied between 1 and 12 micron, and asubstantial proportion of the total volume, about 60%, had grain sizesprimarily in the range 1 to 2 micron. These ultrafine zones wereconcentrated at or close to the surface. From observation, it was foundthat the partially cooled steel presented to the nip was not alreadypartially or wholly transformed, but was still substantially whollyaustenite phase steel. Moreover, it was thought that the austenite toferrite transformation occurred at or very close after the roller nip,suggesting that the mechanism was strain induced transformation. It wasalso observed that there was little or no tendency for the ferritegrains to thereafter coarsen despite the relatively slow rate of coolinginherent in natural air cooling, suggesting that the transformation wassubstantially instantaneous, whereby the grains were locked in positionagainst expansion in size.

FURTHER EXAMPLES

A large number of low, medium and high carbon steels, from bothproduction and laboratory melts, were rolled in a mill. The carboncontents of the steels ranged from 0.036 to 0.77% C, and their fullcompositions are shown in Table 1. The steels were initially roughed to2 mm thick strips and cut into pieces about 100 mm wide and 150 mm long.The strips were reheated to 1250° C. for 10 to 15 minutes in stainlesssteel bags and then air cooled to the desired rolling temperature.Rolling was performed in a single pass, using rolls with a diameter ofapproximately 300 mm. Samples were then allowed to air cool, or coilingwas simulated in a fluidised sand bed at constant temperature for 1 hourfollowed by cooling to room temperature between two kaowool blankets.The rolling entry and exit temperatures were recorded by a pyrometer ateither side of the roll gap. The rolling entry and exit temperatures areshown in Table 2.

The effect of various processing parameters on the microstructure of thestrips was investigated. In addition to the effect of carbon and othercommon alloying elements, the presence of microalloying elements such asNb, Ti and B in some steels was expected to be influential on the finalmicrostructure. The effect of roll entry temperature, reduction, rollspeed, lubrication and feed thickness were also studied. Table 2 showsthe range of experimental conditions investigated for all steels.

Metallographic samples were prepared from the rolled strips usingstandard techniques, and studied using both optical and scanningelectron microscopy. Vickers hardness measurements were made and tensilespecimens were prepared from some strips. Tensile tests were performedon a Sintech tensile machine at a strain rate of 10⁻⁴ S⁻¹.

MICROSTRUCTURES

The steels listed in Table 1 have been divided into plain andmicroalloyed low carbon grades, medium carbon and higher carbon grades.The general feature of all the rolled samples was the presence of anultrafine microstructure, usually consisting of ferrite grains anddiscrete carbide particles in a region near the surface of the samplesand a coarser microstructure in the centre regions. This ultrafineregion generally penetrated to a depth of about 1/4 to 1/3 of the samplethickness (FIG. 6). Individual microstructures are described in moredetail below.

Temperature drops recorded at the exit of the rolling mill ranged from70 to 180° C., although most were in the order of 70 to 100° C. Mostreductions were between 30 and 40%.

PLAIN LOW CARBON STEELS

Four plain low carbon steels were rolled: M06, A06 and 3373 and 3398,with the majority of the experimental conditions being varied for M06and A06.

M06

The effect of roll entry temperature was considered by rolling foursamples at delivery temperatures of 835, 795, 775 and 740° C. (samplesM06-1, 2, 4 and 3 respectively). The first two conditions did not appearto significantly alter the microstructure, with a region of equiaxedferrite of size 1-3 μm penetrating to about 1/4 of the sample depth(FIG. 7A), and a centre region of coarser angular and equiaxed ferriteof size 5-15 μm (FIG. 7B). The third entry temperature resulted in theformation of some proeutectoid ferrite near the surface, possiblyforming on prior austenite grain boundaries. There were, however,ultrafine ferrite grains near the surface as before and a coarse angularstructure in the centre. The lowest delivery temperature produced amicrostructure consisting of large amounts of proeutectoid ferritethroughout the sample (FIG. 7C).

The effect of roll speed was considered by comparing speeds of 0.18,0.27, 0.37 (standard speed) and a 1.0 m/s. The slower roll speeds (M06-5and 6) resulted in a considerably greater temperature loss in the rollgap due to greater contact times with the cold rolls. This produced moreproeutectoid ferrite than for the standard roll speed at a similar entrytemperature (M60-4). At a roll speed of 0.18 m/s, a completely differentmicrostructure was produced. Both the centre and surface of the sampleconsisted of an ultrafine bainitic type microstructure which was highlycrystallographic in nature (FIG. 8A). The surface laths were finer thanthose in the centre. Such a microstructure reflects the largetemperature drop (about 170° C.) that occurred in the roll gap. Thehighest roll speed achieved was 1.0 m/s (M06-16) which resulted in alayered structure, although the ferrite grains in the surface regionswere not ultrafine (FIG. 8B).

Five samples were rolled using a boron nitride spray lubricant on therolls. One sample (M06-8) rolled at 790° C. was reduced 57% andconsisted of large amounts of proeutectoid ferrite throughout the sampleand a phase which appeared as an ultrafine bainite, similar to thatobserved from M06-5. A second sample (M06-10) was rolled at only aslightly higher temperature but reduced only 41%. This sample was notquenched by the rolls to the same degree as the sample M06-5. Itconsisted of a small amount of proeutectoid ferrite, together with arelatively shallow ultrafine (1-3 μm) ferrite region and a coarse (5-15μm) angular ferrite centre. Samples M06-18 and 19 were rolled withlubricant at 800 and 775° C. and again produced slightly differentstructures, with more severely quenched surface regions and lessproeutectoid ferrite. These differences may be due to a variation inlubricant thickness. The application of lubricant to one roll only(M06-17) resulted in a quenched microstructure near the lubricatedsurface and a relatively fine ferrite structure at the opposite surface(FIG. 9). There was little increase in reduction compared with anunlubricated sample (FIG. 9B). Two scaled (ie not reheated in bags)samples were rolled (M06-21 and 22) and resulted in relatively coarseequiaxed ferrite surface grains (up to 10 μm) and coarse centre regions(10-20 μm). The scale was expected to act as a lubricant and although itslightly decreased roll loads and increased total reduction, thepresence of scale did not produce structures similar to those rolledwith the lubricant sprayed onto the rolls. The scale did however act asan insulator and reduced the temperature drop to around 40° C.

The final condition varied for the M06 material was the effect ofcoiling the rolled strip in the fluidised sand bed (M06-15). There wasno apparent change in the surface or centre grain size of sample M06-15,although the carbide distribution was altered by the coiling process(FIG. 10A). It appears that there is a greater proportion of carbides atthe grain boundaries and triple points in the sample that has beencoiled (FIG. 10B).

A06

Conventional A06 was rolled under similar conditions to M06, althoughthe reheat temperature was reduced in some cases. In general,microstructures similar to M06 were obtained, although there was morevariation through the thickness and in the direction of rolling.

Roll entry temperature was varied for samples A06-1, 2, 3 and 8. Thehighest entry temperature of 905° C. was employed for A06-8 and resultedin a reasonably equiaxed structure, with 1 to 4 μm grains near thesurface and coarser grains, up to about 15 μm, in the centre region. Adelivery temperature of 855° C. for A06-2 produced a region of equiaxedferrite of similar depth to sample A06-8, together with a centreconsisting of coarse, angular ferrite grains of various orientations,often greater than 20 μm in length. Decreasing the entry temperature by50° C. (A06-1) produced a similar structure, although there was theappearance of some proeutectoid ferrite. The lowest rolling temperatureof 755° C. (A06-5) produced large amounts of coarse proeutectoidferrite, although the ultrafine surface bands remained.

Roll speed was investigated as a process variable and a similar trend toM06 was observed. A low roll speed of 0.18 m/s (A06-4) produced asimilar structure to the sample rolled at the same temperature (A06-1),although the temperature drop was over 100° C. greater and considerableproeutectoid ferrite was produced. An intermediate speed of 0.27 m/sresulted in an overall coarser microstructure, although this sample(A06-7) was rolled at a higher temperature.

Reducing the reheat temperature to 1050° C. for samples A06-5 and 6significantly reduced the volume fraction of ultrafine grains in thesurface regions and increased the coarseness of the centre grains. Thesample rolled at the higher entry temperature (A06-5) had regions offerrite grains less than about 4 μm in size, but these regions wereisolated and not situated directly near the surface. A lower deliverytemperature (A06-6) produced far fewer regions of ultrafine ferrite, andthere were very coarse, angular grains throughout the wholemicrostructure, extending even to the surface. There was also someevidence of warm worked ferrite grains.

3373

The microstructure of this grade (0.065% C-1% Mn) consisted of a surfacelayer of ultrafine ferrite grains (1-2 μm) penetrating to about 1/4 ofthe sample depth (FIG. 11A), with regions of segregated carbides whichappeared to be aligned in rows. The centre (FIG. 11B) consisted of largevolume fractions of course Widmanstatten or acicular ferrite, withevidence of a second phase, possibly pearlite.

3398

This high Si grade provided some insight into the effect of prioraustenite grain size, as determined largely by reheat temperature, onthe final microstructure. A high reheat temperature of 1250° C. (FIG.12A) resulted in a similar structure to that obtained in the 3373 heat,although the surface layers were not as fine overall and the centreconsisted of coarser, more blocky ferrite grains, with some discretemartensite islands. Carbides were present at the ferrite grainboundaries and were continuous around a large percentage of ferritegrains. Reheating the sample to only 950° C. (3398-2) produced distinctsurface and centre regions as before, however, the surface consisted ofa mixture of ultrafine grains or sub-grains and large, worked ferritegrains (FIG. 12B). The centre consisted of reasonably equiaxed ferrite(about 5 to 10 μm) and discrete carbides and some small islands ofmartensite.

MICROALLOYED LOW CARBON STEELS

Ti Additions

Steel 3403 (0.024% Ti) produced a 1/4 sample depth region of uniformultrafine ferrite grains (FIG. 13A) and a centre region consisting ofangular and some acicular ferrite grains, dispersed carbides anddiscrete islands of martensite (FIG. 13B). A similar steel with 0.20% Moaddition (3404) resulted in a similar structure, although the surfacelayers consisted of even finer ferrite grains (<1-2 μm) (FIG. 14A) andthe ferrite in the centre of the samples was finer and more acicular(FIG. 14B). Once again there were small packets of martensite present.

Higher additions of Ti, such as in welding rod steel (3393 and 3394)resulted in ultrafine ferrite surface layers (FIG. 15A) and extremelyfine acicular ferrite structures in the cental regions (FIG. 15B). Theultrafine ferrite could not be resolved using optical microscopy,however, electron microscopy revealed sub-micron grains. Once again,isolated islands of martensite were scattered throughout the acicularferrite.

Nb Additions

Two conventional steel grades containing both Nb and Ti, XF400 andXF500, were processed and produced similar surface microstructuresconsisting of ferrite grains down to about 1 μm in size, but slightlydifferent centre structures. The central regions of the XF400 sampleconsisted of angular and blocky ferrite grains, which were inconsistentboth in terms of size and shape, ranging from about 5 to 15 μm. TheXF500 sample, however, produced a finer, slightly more uniform acicularferrite microstructure.

Sample 3370 containing 0.037% Nb was used to investigate the effect ofincreased feed thickness, lubrication and coiling after rolling. Thestandard sample with initial thickness of 2 mm (3370-1) consisted of theusual ultrafine ferrite to 1/4 sample depth, together with a mixture ofangular and acicular ferrite in the centre. When the feed thickness wasincreased to 4 mm (sample 3370-2), the grain size in the surface regionswas not quite as fine (up to about 4 μm), and the depth of penetrationwas not as great, probably only reaching about 1/5 sample depth. Thetemperature drop in the roll gap was just over 50° C. Lubrication wasemployed for sample 3370-3 and the temperature drop increased to morethan 140° C., most likely due to the heat conducting effect of thelubrication. The grain size in the surface regions was similar, but lessuniform and the depth of this region had decreased even further. Themicrostructure of the central regions remained essentially similar.Sample 3370-4 (2 mm input thickness) was coiled at 600° C. after rollingat 750° C., which was a lower delivery temperature than for the firstthree samples. The depth of the ultrafine surface regions approached 1/3of the sample thickness, probably the greatest penetration of all thesamples. The grain size in that region was less than 1 μm. The centralregions remained relatively unchanged and so coiling did not appear tosignificantly alter the overall microstructure.

Other Additions

Samples 3607 and 3608 both contained Mo and Ti, with 3608 containing0.002% B. The addition of B did not appear to change the microstructuresignificantly, with both samples consisting of the standard depth ofultrafine grains and angular ferrite grains in the centre. Sample 3608-1had a region right at the surface which was not ultrafine, although thismay have been the result of decarburisation. Steel 3607 was also coiledat 600° C. after rolling (3607-2), however the entry roll temperaturewas 50° C. lower than for 3607-1, making a comparison of the twodifficult. Nevertheless, there was little microstructural change aftercoiling.

Steel 3399 contained 0.48% Cr, and produced a region of 1-2 μm ferritegrains near the surface, and acicular ferrite with a considerable volumefraction of martensite islands in the centre of the strip.

MEDIUM CARBON STEELS

These grades contained about 0.2 to 0.4% C and in some cases Ti, V andB. The plain carbon sample 3374, contained 0.21% C and consisted of asurface region of equiaxed ferrite grains of size 1-3 μm with finecarbides segregated into rows (FIG. 16A). Acicular ferrite was presentin the centre and there was some necklacing of fine ferrite grainsaround prior austenite grains boundaries (FIG. 16B). The second plaincarbon steel (1040) was processed under three conditions; namely rollingat 750 and 700° C. followed by air cooling, and rolling at 750° C. withcoiling at 600° C. All samples had the characteristic ultrafinemicrostructure to a depth of 1/3 sample thickness. In this region, therewas very fine ferrite and a high volume fraction of carbides distributedthroughout (FIG. 17A). Proeutectoid ferrite formed in the centre of thestrips, outlining the prior austenite grain boundaries, however themajority of this region was pearlitic (FIG. 17B). In this case, coilingdid not appear to significantly alter the carbide distribution (FIGS.17C and D).

Samples 3521 (Ti addition) and 3524 (Ti and V additions) were bothprocessed under the same conditions as the 1040 grade. Both compositionshad similar microstructures for almost all conditions. These consistedof ultrafine ferrite grains and carbides in the surface regions,although the carbides appear as finer, more discrete particles for thetwo samples coiled at 600° C. (compare FIGS. 18A and B). The ultrafinegrains were also slightly finer in the samples rolled at lowertemperatures (3521-3 and 3524-3). The center regions consisted ofacicular ferrite grains distributed throughout a pearlitic matrix. Theseacicular structures were generally finer in the sample containing V andwere particularly refined in sample 3524-3.

The final medium carbon steel (3605) contained Ti and B. Itsmicrostructure was similar to the lower carbon samples, 3607 and 3608(alloyed with Ti, Mo and B), although there were more carbides present,particularly in the ultrafine surface regions (FIG. 19A), as would beexpected. A second sample (3605-2) was reheated to only 950° C. beforerolling and similar to sample 3398-2, consisted of relatively coarseworked ferrite grains in the surface regions, together with distinctsmall regions of carbides and ultrafine grains or sub-grains (FIG. 19B).Ferrite grains in the centre regions were reasonably equiaxed. This samematerial was also reheated to both 950 and 1250° C., quenched and etchedfor austenite grain boundaries. The lower reheat produced 10-20 μmgrains, while the higher reheat resulted in grains from 100 to 400 μm insize.

HIGH CARBON STEELS

The two pearlitic steels, 1062 and 1077, were both rolled under the samethree conditions used to process samples 3521, 3524 and 1040. Thereappeared to be little difference between the microstructures producedunder the various conditions. There was again evidence of shearing inthe surface regions of both steels, with ultrafine ferrite grains (lessthan 1 μm in size) and discrete carbides present in these regions (FIG.20). The depth of ultrafine ferrite was, however, less than thatobserved in the low carbon samples, although this may have been due tothe lower reductions achieved (generally 20 to 25%). The centre regionsconsisted of colonies of pearlite, with microstructures similar to thoseexpected in conventionally processed high carbon grades (FIG. 20).

MECHANICAL PROPERTIES

Mechanical properties of all steels are shown in Table 3. The 0.2% proofstress was determined for the higher C steels since there was nodefinite upper or lower yield point. One of the most unusual aspects ofthese results was the flatness of many of the stress-strain curves,especially for the lower C grades. This is reflected in the ratioYS/UTS, which in many cases was close to 1.00. An example of thisabsence of work hardening is shown in the stress-strain curve of sampleA06-8 (FIG. 21), where the maximum stress occurred at the upper yieldpoint. After this, the stress dropped and remained below the initiallevel. The higher C steels did work harden to a much greater extent, inparticular the 1040, 1062 and 1077 commercial grades. A typical curve isshown in FIG. 22 (sample 1062-1).

The results show that very high strengths are achievable with this typeof processing. A plain low carbon steel (M06-9) has obtained a yieldstrength of 590 MPa together with 16% total elongation and A06-8produced twice that elongation with a yield strength of 430 MPa Thethird plain C steel (3373) containing 0.065% C also had excellentproperties: LYS and UTS of 520 and 580 MPa respectively, and totalelongation of 23%. Of the lower C steels, the greatest strengthproperties were obtained in the two welding rods steels (3393 and 3394)with LYS of 745 and 830 MPa. Lowering the reheat temperature in samples3398 and 3605 produced significant strength increments, althoughductility was adversely affected. This is an interesting result giventhe transition from an ultrafine ferrite microstructure after highreheat, to a coarser, worked ferrite structure after low reheat.

The results for M06 rolled under various conditions indicate thatseveral processing factors can influence the final properties. Rollentry temperature (M06- 1,2 and 4) did not significantly change thestrength of the material, although a high rolling temperature producedthe most ductile strip. Coiling after rolling softened the material andincreased elongation, as did rolling at higher speeds (M06-16). The lowreheat (M06-13) produced properties only slightly inferior to the normalhigh reheat strip (M06-14) despite the formation of a completelydifferent microstructure. As expected from the microstructures, samplesprocessed with lubricated rolls produced much higher strengths than thescaled samples. Not surprisingly, the relatively coarse microstructureof scaled sample M06-21 resulted in by far the softest strip of allmaterials tested.

The higher C grades displayed continuous yielding and so a proof stresswas measured instead of LYS. These steels displayed greater workhardening than the lower C samples and produced some very high strengthvalues. Due to its Ti and V additions, sample 3524 obtained proof andtensile strengths higher than 1040 grade despite the lower C content,along with equivalent ductility. The pearlitic steels 1062 and 1077 hadstrengths greater than those usually obtained under industrialconditions (in bar form), although total elongations were lower. In allmedium and high C grade steels, coiling at 600° C. decreased both PS andUTS (by over 100 MPa in the case of 1077) but had little effect onductility. With the exception of heat 3524, a decrease in roll entrytemperature by 50° C. produced a strength increment of at least 30 MPa.

At the present time the exact mechanism by which the transformation ofthe austenite phase steel to an ultrafine microstructure occurs is notfully understood. It is theorised that by reducing the grain boundaryarea in the austenite phase and then by applying a pre-cooling, thedriving force to cause the transformation to a ferrite phase becomesvery high. However, there is insufficient grain boundary area to achievenucleation. Therefore by treating the steel (ie deforming the steel)while in the austenite phase and before any substantial transformationhas commenced a strain induced homogenous transformation to the ferritephase occurs. This homogenous transformation occurs rapidly and theferrite grain size is extremely small.

The transformation to fine ferrite grains is ascribed to a homogenoustransformation rather than to a dynamic recrystallisation of thetransformed ferrite as taught by U.S. Pat. No. 4,466,842. TABLE 1

    __________________________________________________________________________    COMPOSITION OF ALL STEELS INVESTIGATED (IN WT %)                                        Steel                                                                 ID           C       P        Mn     Si       S       Cr      Mo                                                         AI        Nb     Ti       V                                                       B        N                   __________________________________________________________________________        0.036   94                                                                       0.017                                                                            1.39                                                                             0.62                                                                             0.012                                                                            0.019                                                                            0.006                                                                            0.013                                                                            <0.005                                                                            0.12                                                                              0.005                                                                             <0.0003                                                                            0.0056                             3607    0.043   0.018    1.67   0.21     0.011  0.008   0.26    0.034                                                      0.022  0.016    <0.003                                                      <0.0003  0.0035                    3608    0.044   0.019    1.73   0.22     0.01    0.009   0.26    0.03                                                       0.022  0.017    <0.003                                                     0.0019   0.0035                    A06     0.06    0.013    0.21   0.005    0.011  0.017   0.002   0.04                                                              0.0036                    3373    0.065   0.018    0.99   0.27     0.006   0.004   0.002   0.03                                                       <0.005 <0.003   <0.003                                                     <0.0003  0.0018                    M06     0.075   0.015    0.5l   0.26     0.008   0.018   0.002   0.005                                                                        0.003                                                     3393    0.08    0.018                                                        1.34   0.63     0.013                                                         0.019   0.007   0.017                                                         <0.005 0.16     0.006                                                         0.0003   0.0077                    XF400   0.09    0.015    0.71   0.015    0.005   0.02    0.002   0.03                                                       0.027  0.025                                                                        0.0034                    3403    0.10    0.017    1.50   0.76     0.011  0.017   0.005   0.026                                                      <0.005 0.024    <0.003                                                      0.0004   0.0047                    XF500   0.10    0.018    1.47   0.08     0.005   0.037   0.004   .0.03                                                      0.045  0.031    0.004                                                      0.0005   0.0063                    3370    0.105   0.005    0.86   0.29     0.005   0.004   0.002   0.019                                                      0.037  0.006    <0.003                                                     <0.0003  0.0041                    3398    0.105   0.018    1.38   1.40     0.011  0.017   0.004   0.026                                                      <0.005 0.004    <0.003                                                      <0.0003  0.0038                    3399    0.105   0.018    1.38   0.16     0.011  0.48    0.004   0.024                                                      <0.005 <0.003   <0.003                                                      <0.0003  0.0041                    3404    0.105   0.017    1.50   0.31     0.012   0.017   0.20    0.022                                                      <0.005 0.022    <0.003                                                     <0.0003  0.0051                    3605    0.175   0.019    1.68   0.20     0.013   0.008   0.003   0.038                                                      <0.005 0.017    <0.003                                                     0.0016   0.0043                    3374    0.21    0.02     0.99   0.29     0.006   0.003   0.002   0.033                                                      <0.005 <0.003   <0.003                                                     <0.0003  0.0025                    3524    0.27    0.005    1.67   0.36     0.034   0.008   0.003   0.036                                                      <0.005 0.014    0.12                                                       <0.0003  0.014                     3521    0.29    0.018    0.85   0.21     0.008   0.014   0.009   0.037                                                      <0.005 0.02     0.003                                                      0.0006   0.0023                    1040    0.38    0.019    0.76   0.20     0.008   0.022   0.003   0.04                                                       <0.005 <0.002   <0.002                                                     <0.0003  0.004                     1062    0.63    0.023    0.75   0.22     0.019   0.04    0.01    0.03                                                              0.003                    1077    0.77    0.018    0.71   0.184    0.007   0.01    0.04    0.01                                                              0.004                  __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    PROCESSING CONDITIONS FOR ALL STRIPS                                               Reheat    Roll/                                                                              Entry                                                                            Exit     Total                                           Sample       Temp      Roll Speed  Sample      Temp      Temp     Post                                      Roll   Reduction                                Name            (° C.)      (m/sec)          Condition                                               (° C.)      (° C.)                                              Cooling           (%)                         __________________________________________________________________________         1250                                                                              0.37       835                                                                              710                                                                               Air  41                                              M06-2      1250      0.37                   795       685      Air                                           35                                             M06-3      1250      0.37                   740       675      Air                                           35                                             M06-4       1250      0.37                   775       685      Air                                           35                                            M06-5      1250      0.18                   775       605      Air                                           30                                             M06-6      1250      0.27                   785       660      Air                                           35                                             M06-8      1250      0.37     Lubricated  790       630      Air                                            57                                              M06-9      1250      0.27                   800       670      Air                                           35                                             M06-10     1250      0.37     Lubricated  800       690      Air                                            41                                              M06-13     950       0.30                   775                  Air                                           29                                           M06-15     1250      0.30                   790       720      Coil                                         650° C. 29                               M06-16     1250      1.0                    800       715      Air                                           29                                             M06-17     1250      0.30     Lub 1 Roll  800                  Air                                           31                                             M06-18     1250      0.30     Lubricated  800                  Air                                           37                                             M06-19     1250      0.30     Lubricated  775       <670     Air                                            40                                              M06-21     1250      0.30     Scaled      810       770      Air                                            31                                              M06-22     1250      0.30     Scaled      780       745      Air                                            31                                              A06-1      1250      0.37                   800       720      Air                                           40                                             A06-2       1250      0.37                   855       745      Air                                           45                                            A06-3       1250      0.37                   755       685      Air                                           35                                            A06-4       1250      0.18                   810       625      Air                                           33                                            A06-5      1050      0.37                   805       700      Air                                           35                                             A06-6      1050      0.37                   750       650      Air                                           33                                             A06-7      1250      0.27                   900       705      Air                                           43                                             A06-8      1250      0.37                   905       760      Air                                           45                                             3370-1     1250      0.37                   800       695      Air                                           40                                             3370-2     1250      0.37                   800       745      Air                                           49                                             3370-3     1250      0.37     Lubricated  805       660      Air                                            54                                              3370-4      1250      0.30                   750       670      Coil                                        600° C. 44                               3373-1     1250      0.37                   800       675      Air                                           35                                             3374-1     1250      0.37                   755       690      Air                                           40                                             3393-1     1250      0.37                   770       680      Air                                           40                                             3394-1     1250      0.37                   800       680      Air                                           41                                             3398-1     1250      0.37                   775       690      Air                                           40                                             3398-2     950       0.30                   775       635      Air                                           30                                             3399-1     1250      0.37                   800       675      Air                                           37                                             3403-1     1250      0.37                   810       700      Air                                           38                                             3404-1     1250      0.37                   765       650       Air                                          37                                             3605-1     1250      0.37                   765       695       Air                                          41                                             3605-2     950       0.30                   775       660       Air                                          31                                             3607-1     1250      0.37                   795       690       Air                                          35                                             3607-2     1250      0.30                   750       660       CoiI                                        600° C.  33                              3608-1     1250      0.37                   800       715       Air                                          41                                             XF400-1    1250      0.37                   800       700       Air                                          38                                             XF500-1    1250      0.37                   775       675       Air                                          41                                             3521-1     1250      0.30                730       660       Coil                                           600° C.  30                              3521-2     1250      0.30                750       660       Air                                            34                                              3521-3     1250      0.30                705       625       Air                                            30                                              3524-1     1250      0.30                750       --         Air                                           29                                              3524-2     1250      0.30                750       --         Coil                                          600° C.  29                              3524-3     1250      0.30                700       --         Air                                           29                                              1040-1     1250      0.30                750       615       CoiI                                           600° C.  26                              1040-2     1250      0.30                750       615       Air                                            26                                              1040-3     1250      0.30                700       600       Air                                            24                                              1062-1     1250      0.30                760       655       Coil                                           600° C.  26                              1062-2     1250      0.30                755       640       Air                                            30                                              1062-3     1250      0.30                690       600       Air                                            26                                              1077-1     1250      0.30                735       610       Coii                                           600° C.  21                              1077-2     1250      0.30                755       620       Air                                            26                                              1077-3     1250      0.30                700       580       Air                                            21                                            __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                                                             MECHANICAL PROPERTIES OF ALL STEELS       (SPECIMENS FROM SAMPLE 3608 REACHED STRESSES                                  OF 517 AND 538 MP.sub.A BEFORE FAILING PREMATURELY)                                     LYS      0.2% PS                                                                              UTS    LYS/UTS                                                                              TE (%)                                 Name           (MPa)          (MPa)        (MPa)        PS/UTS                                                       (75 mm)                              ______________________________________                                               745               748    1.00   11                                       3607-1         495                          507          0.98                                                      17                                       3607-2         446                           494          0.90                                                     19                                       A06-8          432                           432          1.00                                                     32                                       3373-1         520                           580          0.90                                                     23                                       M06-1          490                           507          0.97                                                     25                                       M06-2          471                           497          0.95                                                     13                                       M06-4          502                           520          0.97                                                     19                                       M06-9          589                           589          1.00                                                     16                                       M06-10         540                           552          0.98                                                     17                                       M06-11         481                           523          0.92                                                     22                                       M06-13         481                           538          0.89                                                     14                                       M06-15         435                           472          0.92                                                     22                                       M06-16         428                           490          0.87                                                     23                                       M06-18         566                           607          0.93                                                     14                                       M06-21         306                           360          0.85                                                     16                                       3393-1         830                           874          0.95                                                     16                                       XF400-1        576                           576          1.00                                                     11                                       3403-1         535                           609          0.88                                                     26                                       XF500-1        670                           672          1.00                                                     11                                       3370-1         603                           617          0.98                                                     17                                       33704          633                           633          1.00                                                     8                                        3398-1         580                           634          0.91                                                     20                                       3398-2         662                           720          0.92                                                     11                                       3399-1         520                           605          0.86                                                     22                                       3404-1         530                           695          0.76                                                     20                                       3605-1         490                           499          0.98                                                     23                                       3605-2         521                           557          0.94                                                     13                                       3374-1         500                           505          0.99                                                     26                                       3524-1                          742          873          0.85                                                     17                                       3524-2                          696          792          0.88                                                     14                                       3524-3                          745          840          0.89                                                     12                                       3521-1                          545          607          0.90                                                     18                                       3521-2                          581           631          0.92                                                     18                                      3521-3                          611          664          0.92                                                     16                                       1040-1                          517          731          0.71                                                     13                                       1040-2                          542          733          0.74                                                     14                                       1040-3                          575          768          0.75                                                     13                                       1062-1                          573          864          0.66                                                     8                                        1062-2                          613          875          0.70                                                     8                                        1062-3                          671          945          0.71                                                     9                                        1077-1                          627          959          0.65                                                     8                                        1077-2                          729          1067         0.68                                                     7                                        1077-3                         777          1094         0.71               ______________________________________                                                                               6                                  

What is claimed is:
 1. A method of producing a steel having one or morezones of ultrafine microstructure comprising treating an austenite phasesteel having a mean austenite grain size greater than 50 microns beforeany substantial transformation has commenced so as to induce a rapidsubstantially complete transformation to an ultrafine microstructure inone or more zones of the microstructure.
 2. A method of producing asteel having one or more zones of ultrafine microstructure comprisingheating a steel to austenitise the steel, pre-cooling the austenitephase steel to produce an austenite phase steel having a mean austenitegrain size greater than 50 microns, and treating this pre-cooledaustenite phase steel before any substantial transformation hascommenced so as to induce a rapid substantially complete transformationto an ultrafine microstructure in one or more zones of themicrostructure.
 3. A method of producing a steel having one or morezones of ultrafine microstructure comprising partially pre-coolingfreshly cast austenite phase steel to produce an austenite phase steelhaving a mean austenite phase steel before any substantialtransformation has commenced so as to induce a rapid substantiallycomplete transformation to an ultrafine microstructure in one or morezones of the microstructure.
 4. A method according to claim 2 whereinsaid pre-cooling of the austenite phase steel is by natural air, forcedair or water cooling at a rate in the range 50 to 2000 K°/min.
 5. Amethod according to claim 1 wherein the treatment applied to theaustenite phase steel is a deformation.
 6. A method according to claim 5wherein the deformation is performed at a temperature in the range of600° C. to 950° C.
 7. A method according to claim 5 wherein, forproducing a low carbon steel, the deformation is performed at atemperature in the range of 700° to 950° C.
 8. A method of producing asteel having one or more zones of ultrafine microstructure comprisingdeforming an austenite phase steel having a mean austenite grain sizegreater than 50 microns before any substantial transformation hascommenced to develop a strain profile or strain gradient across thestructure of the steel so as to induce a rapid substantially completetransformation to an ultrafine microstructure in one or more zones ofthe microstructure.
 9. A method according to claim 8 wherein the zone ofthe ultrafine microstructure comprises a whole cross-section of thestructure.
 10. A method according to claim 8 wherein the zones of theultrafine microstructure comprises a surface layer or layers of thesteel.
 11. A method according to claim 8 wherein the strain profilecomprises a relatively higher strain in a surface layer or layers of thesteel and a relatively lower strain in the core.
 12. A method accordingto claim 11 wherein the strain inhomogeneity is enhanced by havingfriction conditions existing between the surface of the steel beingdeformed and the means by which the steel is deformed.
 13. A methodaccording to claim 5 wherein the deformation comprises passing the steelbetween a pair of contra-rotating rolls effective to reduce a thicknessdimension of the steel by a proportion in the range 20 to 70%.
 14. Amethod according to claim 13 wherein the thickness dimension of thesteel is reduced by a proportion of 30 to 60%.
 15. A method according toclaim 13 where only a single pass of the steel is performed to achievedeformation.
 16. A method according to claim 13 wherein the rollingspeed is in the range 0.1 to 5.0 m/s.
 17. A method according to claim 13when dependent on claim 8 wherein the ratio of the rolling arc (L_(d))of the rolls to nip gap or rolling thickness (H_(m)) is greater than 10.18. A method according to claim 2 wherein the steel is heated to atemperature between 1000° C. to 1400° C.
 19. A method according to claim2 wherein the steel is heated to a temperature in the range 1100° C. to1300° C.
 20. A method according to claim 1 wherein the steel is cooledafter the transformation.
 21. A method according to claim 1 wherein thesteel is pretreated in a manner effective to reduce or substantiallyeliminate grain boundary nucleation of grains, whereby to facilitatesaid rapid substantially complete transformation.
 22. A method accordingto claim 21 wherein the pretreatment comprises a pretreatment to enlargethe mean austenite grain size of a selected steel or may alternativelyor additionally comprise a chemical treatment selected to reduce grainboundary reactivity.
 23. A method according to claim 21 wherein thepretreatment entails a pre-cooling of the steel from a highertemperature.
 24. A method according to claim 5 wherein said pre-coolingof the austenite phase steel is by natural air, forced air or watercooling at a rate in the range of 50 to 2000 K°/min.
 25. A methodaccording to claim 8 wherein the deformation comprises passing the steelbetween a pair of contra-rotating rolls effective to reduce a thicknessdimension of the steel by a proportion in the range 20 to 70%.
 26. Amethod according to claim 25 wherein the thickness dimension of thesteel is reduced by a proportion of 30 to 60%.
 27. A method according toclaim 25 where only a single pass of the steel is performed to achievedeformation.
 28. A method according to claim 25 wherein the rollingspeed is in the range 0.1 to 5.0 m/s.
 29. A method according to claim 25wherein the ratio of the rolling arc (L_(d)) of the rolls to nip gap orrolling thickness (H_(m)) is greater than
 10. 30. A method according toclaim 2 wherein the steel is cooled after the transformation.
 31. Amethod according to claim 3 wherein the steel is cooled after thetransformation.
 32. A method according to claim 5 wherein the steel iscooled after the transformation.
 33. A method according to claim 8wherein the steel is cooled after the transformation.
 34. A methodaccording to claim 10 wherein the steel is cooled after thetransformation.
 35. A method according to claim 2 wherein the steel ispretreated in a manner effective to reduce or substantially eliminategrain boundary nucleation of grains, thereby facilitating said rapidsubstantially complete transformation.
 36. A method according to claim 3wherein the steel is pretreated in a manner effective to reduce orsubstantially eliminate grain boundary nucleation of grains, therebyfacilitating said rapid substantially complete transformation.
 37. Amethod according to claim 5 wherein the steel is pretreated in a mannereffective to reduce or substantially eliminate grain boundary nucleationof grains, thereby facilitating said rapid substantially completetransformation.
 38. A method according to claim 8 wherein the steel ispretreated in a manner effective to reduce or substantially eliminategrain boundary nucleation of grains, thereby facilitating said rapidsubstantially complete transformation.
 39. A method according to claim10 wherein the steel is pretreated in a manner effective to reduce orsubstantially eliminate grain boundary nucleation of grains, therebyfacilitating said rapid substantially complete transformation.